Optimistic metadataserver

[sargun]

Refactor metadataserver to enable optimistic IAM fetch

• Cleanup titus metadata service init
• Make the metadata server us the local region's STS service

This also makes it so that when the metadata server is hit, and it fetches credentials, it'll keep refreshing them automatically, rather than requiring force refresh.

[coveralls]

Pull Request Test Coverage Report for Build 2648

• 11 of 163 (6.75%) changed or added relevant lines in 5 files are covered.
• 20 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.1%) to 22.341%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
executor/runtime/docker/docker.go	0	5	0.0%
executor/runtime/types/types.go	0	6	0.0%
metadataserver/iam.go	9	65	13.85%
cmd/titus-metadata-service/main.go	0	85	0.0%

Files with Coverage Reduction	New Missed Lines	%
cmd/titus-metadata-service/main.go	1	0.0%
metadataserver/iam.go	4	18.6%
cmd/titus-executor/main.go	15	4.76%

Totals
Change from base Build 2593:	-0.1%
Covered Lines:	2481
Relevant Lines:	11105

---

💛 - Coveralls

[codecov]

Codecov Report

Merging #207 into master will decrease coverage by 0.17%.
The diff coverage is 7.74%.

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
- Coverage   32.35%   32.18%   -0.17%     
==========================================
  Files          68       68              
  Lines        8648     8703      +55     
==========================================
+ Hits         2798     2801       +3     
- Misses       5508     5557      +49     
- Partials      342      345       +3

Impacted Files	Coverage Δ
cmd/titus-metadata-service/main.go	0% <0%> (ø)	⬆️
executor/runtime/docker/docker.go	51.85% <0%> (-0.19%)	⬇️
metadataserver/iam.go	15.21% <10.2%> (-0.38%)	⬇️
metadataserver/server.go	78.57% <100%> (ø)	⬆️
executor/runtime/types/types.go	63.44% <80%> (+0.59%)	⬆️
cmd/titus-executor/main.go	4.61% <0%> (+0.5%)	⬆️

[rgulewich]

Copy and pasta?